For everyone who is saying the scout is useless. Take the winger and fully upgrade its firing speed and ammo. It'll rip through anything, stopped the bomb getting set off last minute so many times with it.

Get the Mini sentry first as an engineer. extra defense is much better. also if more engineers more sentry busters will come. so 1 Good engineer is much better than having a bunch of bad/good engineers.

Demoman's sticky launcher is god-tier with just a few upgrades. At the start of a wave, ALWAYS HAVE SOMEONE GO KRITZ MEDIC. Have them uber you and place 8 critical stickies. This will at least destroy the first group, or do a chunk of damage to a tank. 8 crit stickies placed together can even take out one or more giants. When the medic's done kritzing they can simply kill themselves and respawn as medic with normal uber, or as their normal class.

The normal bots could only make it a fraction of a the way through the map, giant bots only survived for so much longer, and Sentry Buster Bots had their shit handed to them literally the second they set foot on the map since we all went batshit insane on them with our wranglers all at once.

with tanks, before the round starts, have most of your team demomen and one medic with a kritzkrieg (even if he doesn't want to play medic) and let demomen lay down stickies in the tank path while the medic ubers them so they're all crits. then, after that's all done, the medic can choose whatever class

6 engineers. Two with wranglers, build just their sentry guns, and take on all the defense. Two with short circuits, can build teleporters if needed, shoot down rockets and nades, and take out the ones that make it past the wrangler'd sentries. And finally, two that build just dispensers and take care of the repairs, upgrades, and giving the sentries ammo. Since everybody built just one building, sentry busters will be practically no threat. The rest of the engineers that never built sentries could possibly put some down in other spots, but they don't take much priority.

Scout picks up all money and kites enemies, and kills the spy when he appears
Engineer covers chokepoints
Pyro deals a lot of damage and airblasts bigger enemies
Demo stickyspams, uses most of the kritzcharge
Medic heals and builds charge
Heavy deals a lot of damage

Before each round, use the kritzcharge to lay stickies every here and there (you'll have another charge in a couple of seconds anyway).

(Optional) Use voice communication, say where the bigger enemies are, if there is a spy around etc.

scouts job is to get all dat money, supposedly a fully upgraded clipsize/damage/reload force of nature is unbelievably godly but i havent had a chance to try it yet. also mad milk for the slow and sandman because with upgrades you can carry like 4 balls

soldier is the all-arounder, max out your banner (dont use shotty banner helps more imo) and then reload speed/clip size/ammo/damage of your launcher

pyro needs health on kill, once you have it on your flamer buff your ammo count and damage and then trogdor everything to death

demo should carry scottish resistance and ammo/reload speed, giants are so predictable you can hit them with like 20 stickies and make them die horribly, i've heard the loch 'n load is good but idk

demoknights are best with the zatoichi, make sure you get an uber canteen and upgrade crits for kill and then defensive upgrades after, proceed to utterly demolish waves (only get hp per kill if you aren't using the zatoichi)

heavy's best with brass beast, just max damage/ammo/fire rate/etc and mow down everything

engie should be self-explanatory

medics deciding between kritz and uber depends on your teammates but in general upgrade your uber charge/duration

snipers first duty is to max jarate and hurl it on everything remotely dangerous, after that max rifle reload/damage/ammo and just rapidfire noscope everything to death forever

spy is probably the least useful, remember your disguise fools all the robots 100% of the time, max knife armor penetration/attack speed, wait for giants to appear, shove your knife up their butt until they die

I played Heavy on the Mann Co map. An Engineer built a sentry on top of the stacked wood on the left balcony of the building facing the Robot spawning area. Right below his sentry, he built a dispenser. I had put some points into blast damage resistance (Because Soldier waves are fucking devastating otherwise) and I jumped up and stood next to his sentry.

I did a great job of assisting my teammates below, and every now and then when I'd start getting random crits I would mow down entire groups with ease. After a while I could afford the "destroy projectiles" upgrade, and neither Soldier/Demomen groups nor Giant soldiers ever had a chance against me or the Engineer's sentry. Even more fun was when I filled my canteen with Critical hits, now the ability to mow down large groups, Giant enemies, and bring the Tankers down to half health, was all in the grasp of my hands.